Hi Novella, welcome to the forum!
Since you plan to move the plants into larger pots soon, it won't make much difference if you add soil now to the smaller pots. The roots will not have time to grow into the new soil before you transplant, so there's no point to doing it.
If your 2 new plants arrived without showing any signs of stress like wilted leaves, then you do not have to wait more than a couple of days before repotting. We highly recommend you look over the section of the HVH web site called Potting/Planting as well as the directions on the Care sheet that came with the hibiscus. That web page is at
http://www.hiddenvalleyhibiscus.com/care/potting.htmWhen you repot set the root ball deep enough so that the new soil covers the "Cube" and the main stem of the plant has plenty of support so it does not wobble. If it will be in a windy area you can also use one of the bamboo stakes to give the main stem support until it is strongly established and does not need the stake any longer. To do that insert the stake about a half inch away from the main stem so that it is pointing straight up. Attach it to the main stem at one or two points using the green twist ties that also were part of the packaging. Allow some space between the main stem and the bamboo so the stem can grow and air can circulate.
If you have any other questions, please let us know.
Charlie